Gas discharge lasers or gas lasers of the most diverse kind are known, such as CO.sub.2 lasers, N.sub.2 lasers, and excimer lasers. The laser active medium (i.e. a medium with so-called population inversion) in those cases is generated by gas discharge. It is likewise known to employ lasers of the above kind in pulsed or continuous wave (cw) operation.
EP 0 374 287 A1 discloses a vacuum system for a gas discharge laser, comprising a pump assembly which consists of two pumps. Its purpose, however, is to purify the laser gas and reduce the requirement for make-up gas.
The gas discharge always is effected with a certain gas mixture. In the case of the CO.sub.2 laser, for instance, the gas discharge chamber essentially contains carbon dioxide. Chemical reactions take place in the plasma produced by the gas discharge, whereby the working gas of the laser is "consumed". Chemical reactions also may take place at the walls of the gas discharge chamber. Finally, also contaminations in the gas (e.g. caused by desorption from the walls) may lead to chemical reactions with the working gas of the laser, whereby the working gas is consumed and/or undesirable reaction products are formed which may impair the laser performance. Reaction products also may deposit as contaminants on the optical component parts of the laser.
Prior to the entry of the laser working gases (i.e CO.sub.2, N.sub.2, or precious gases and halogens, for example) into the gas discharge chamber of the laser, the latter must be evacuated before laser operation begins. That requires some kind of arrangement of pumps. However, the need for a pump assembly exists not only before the lasing starts but continues to exist during operation of the laser. It is known in the art to make up the laser gas continuously during operation of the laser because of the "consumption" of the working gas of the laser and the undesirable contaminations mentioned above. This means that normally fresh working gas is introduced continuously (or batchwise) into the laser cavity. For instance, a CO.sub.2 laser typically receives up to some 100 liters of fresh laser-gasmix per hour. A corresponding quantity of gas must be pumped off constantly in view of the fact that the gas pressure in the gas discharge chamber, at the same time, must be kept substantially constant.
Hydrocarbons (oil) are a particularly undesirable contamination in the gas of the gas discharge chamber of the laser. It is especially in the gas discharge that the hydrocarbons react and reaction products accumulate on the optical members of the laser resonator, in particular the mirrors, whereby the laser performance is reduced.
Rotary vane pumps, with oil lubrication, are known in the art as pump assemblies for gas discharge lasers, especially CO.sub.2 lasers. Their rotary vane is sealed by oil with respect to the pump wall so as to increase the pumping capacity and prolong the service life of the pump. The service life of the pump is understood to be the length of time for which a pump typically can run between two maintenance intervals. In other words the service life is the typical maintenance-free working period of a pump. The service life and the pumping performance of a rotary vane pump lubricated with oil are relatively high. Yet the pump lubricating oil can get into the gas discharge chamber of the laser where it may have the disadvantageous effects described above. Oil traps can be provided between the pump and the cavity in order to prevent hydrocarbons from penetrating into the gas discharge chamber, they may be embodied by a zeolite screen or a cooling trap which may operate with liquid nitrogen, for instance. Such measures, however, are costly and require intensive maintenance, and that is not desirable when lasers are put to industrial use.
It is likewise known, in principle, to operate rotary vane pumps without oil. Such pumps then are referred to as "operating dry". Dry-operating rotary vane pumps do have the advantage of not involving any hydrocarbons which may enter into the gas discharge chamber of the laser, because of the operation without oil. However, they suffer the disadvantage of having a relatively shorter service life. On the other hand, the reduction in suction performance of a pump operating dry, as compared to a rotary vane pump with oil lubrication, is rather small.
As already mentioned, undesirable gases must be removed entirely from the discharge chamber before starting a gas discharge laser. This evacuation of the chamber or cavity requires a relatively great suction capacity if unpracticably long pumping times are to be avoided. On the other hand, stationary operation of the laser requires only a relatively small suction capacity to pump off quantities of gas in correspondence with the make-up gas supplied. Oil-free diaphragm pumps are sufficient to meet this demand for minor suction performance.